LDO and Linear Regulator

Hi,

with a look into the linear regulators datasheet you can determine whether an LDO which is more expensive may be replaced or not. "Normal" regulators have a droop voltage UDR = UINmin-UOUT of some volts, LDO`s are designed for droop voltages of only some tens of millivolts.

Take your minimum output voltage of your supply. It has to be always greater than the maximum (worst case) droop plus the regulators output voltage.

Normally, the efficiency of linear regulators is rather bad - LDO`s are are built for better efficiency because their losses are low determined by UDR x IOUT, but only if your supply voltage is really low nearby the regulators minimum voltage. So, with great input voltages the efficiency of LDOs is equally bad - the only advantage is that your application will still function at lower voltages where a normal regulator has already clipped the output.

Regards Uwe

Dear friend!

I add to #1

A constant voltage-drop across the series-pass element is ensured.

This aids in controlling temperature of series-pass element to lowest possible.

Regards

Haajee, let me add the following:

"A constant voltage-drop across the series-pass element is ensured."

Thats not exactly the LDO function. A constant voltage-drop is only ensured if the incoming voltage is constant, otherwise it will float exactly with the input. With a constant voltage drop you will never get a constant output if your input changes.

"This aids in controlling temperature of series-pass element to lowest possible."

Thats only true if the input voltage is the absolute minimum input voltage consisting of
Udropmax + Uout.

So both cases are very special cases which are a little bit unusual for a "regulator"-application where you mostly have a, more or less, varying input voltage which has to be made into a constant output voltage. The more input voltage you have the more drop-voltage appears and the more temperature appears - independant on whether you use a "normal" regulator or an LDO.

Regards Uwe

In simple terms the input of a Low Drop Out (LDO) regulator is normally only required to be a few 10's of mV's above it's specified output, because of the stringent design rules encompassed, the quiescent current of the device is usually very small making it ideal for low power battery applications. The thirstier Linear types require a higher input to output voltage ratio and consume far more current. If you have no need to be power critical and have ample power supply then a conventional linear device will suffice...